Both git and mercurial (and darcs for that matter) are huge improvements over centralized SCMs like SVN. You could argue for days which one to use, and won't get any conclusive results.

This. git and Mercurial are pretty much equivalent.

I wouldn't use Darcs, because it has (had?) huge performance problems with large repositories, and doesn't seem to have enough developers. There's also Bazaar, but it is terrible. (Emacs adopted it some time ago, and someone quickly created a git gateway.)